Conventionally, optical transmission systems multiplex client signals, divide the multiplexed signals among slots on the optical signal transmission side and transmit the signals; and on the optical signal receiving side, restore the received signals to the original client signals and transmit the signals to the respective clients. For example, an optical transport network (OTN) is one optical transmission technological specification. Standardization of OTNs is in progress by the International Telecommunication Union-Telecommunication sector (ITU-T) and the Institute of Electrical and Electronic Engineers (IEEE).
In an OTN, a large variety of client signals can be accommodated by an optical channel data unit (ODU) by a mapping method called Generic Mapping Procedure (GMP). An OTN frame accommodating various types of client signals is multiplexed at a higher layer and transmitted. In this case, the higher layer band is divided into multiple bands. For example, if a higher layer is an optical channel transport unit 2 (OTU2), a 10-G bit/second band is divided into eight 1.25-G bit/second bands. In this case, eight tributary slots (TS) can be freely allocated to each OTN frame (ODTU2.ts). This allocation is indicated in an overhead called multiplex structure identifier (MSI), which indicates the divided band structure.
For example, in an optical channel payload unit 4 (OPU4), 80 payload structure identifiers (PSIs) in the MSI are provided. Each PSI has a length of 1 byte (8 bits), where the highest bit indicates “in-use (Allocated)” or “available (Unallocated)” and the lower 7 bits indicate 1 to 80 tributary port numbers.
For example, in an optical channel payload unit 3 (OPU3), 32 PSIs are provided in the MSI. Each PSI has a length of 1 byte (8 bits), where the upper 2 bits indicate the “type of optical channel data tributary unit (ODTU)” or “available” and the lower 6 bits indicate 1 to 32 tributary port numbers.
For example, in an optical channel payload unit 2 (OPU2), 8 PSIs are provided in the MSI. Each PSI has a length of 1 byte (8 bits), where the upper 2 bits indicate the “type of ODTU”, “reserved”, or “available” and the lower 6 bits indicate 1 to 8 tributary port numbers. Under ITU-T standards, a tributary port number is a logic port number and consequently, there are no restrictions concerning which tributary slot is allocated which tributary port number.
Further, in an OTN, if the MSI value set on the transmission side and on the reception side differ, detection as mismatch is prescribed. According to ITU-T, at the reception-side apparatus, the received MSI value (i.e., the MSI value set on the transmission side) and the expected MSI value (i.e., the MSI value set on the reception side) are compared according to tributary slot, and if the values differ, the detection of mismatch alarm according to tributary slot is defined. For reference, see ITU-T G.709/Y.1331 (12/2009), “Interfaces for the Optical Transport Network(OTN)”, for example.
Nonetheless, presently, in the prescribed ITU-T standard, the handling of concatenated signals of logic tributary ports is not sufficiently considered and consequently, there are cases when the detection of MSI mismatches prescribed by standards for the detection of mismatching communicated MSIs cannot be detected. In other words, in an OTN, even if the concatenated tributary slot count for the transmission-side MSI and for the reception-side MSI differ, there are cases when the MSI mismatch alarm cannot be detected.
FIG. 25 is a table of one example of MSI values received at the reception-side. FIG. 26 is a table of one example of MSI values expected at the reception-side. For example, in the example depicted in FIG. 25, the transmission-side is set such that a 4-Gbps fiber channel (4GFC) is mapped to ODTU2.4 and allocated to tributary slots 4 to 7. In the example depicted in FIG. 26, the transmission-side is set such that a 2-Gbps fiber channel (2GFC) is mapped to ODTU2.2 and allocated to tributary slots 4 and 5.
In the examples depicted in FIG. 25 and FIG. 26, among the received MSI values and the expected MSI values, the setting of tributary slots 4 and 5 coincide and therefore, the reception-side apparatus does not detect the MSI mismatch alarm. Further, among the expected MSI values, tributary slots 6 and 7 are set as available and therefore, even though settings for the received MSI values and the expected MSI values differ, the reception-side apparatus does not detect the MSI mismatch alarm. In this case, concerning tributary slots 6 and 7, the reception-side apparatus detects a loss of frame (LOF) alarm, which is unsuitable.
On the other hand, a synchronous optical network (SONET) uses concatenated information in the overhead. In an OTN, it is conceivable for concatenated information similar to SONET to be stored to available overhead. However, in OPU4, 80 units of concatenated information have to be defined for the tributary slots, which makes processing complicated and consequently, is unrealistic.